Five Misconceptions in Genetics
By Crystal Jeter
Students may bring a variety of misconceptions with them when they enter a study of genetics. Watch your classroom for the 5 common misconceptions listed below. If you find any of them, just use the simple explanations—also provided below—to dispel your students’ incorrect notions.
- One set of alleles is responsible for determining each trait, and there are only 2 different alleles (dominant and recessive) for each gene. After learning about simple Mendelian inheritance and sex-linked traits, students often think that it is possible to model all traits so easily and predictably. In humans, at least 3 different genes are associated with eye color. Coat color in cats is controlled by at least 6 genes. Furthermore, the number of particular alleles inherited determines the expression of some characteristics; for example, the number of alleles—that you inherit from each parent—that code for production of melanin may partially determine your hair color. Inheritance of more of the alleles may lead to darker hair, while inheritance of fewer may lead to lighter hair. For traits that show a Mendelian pattern of inheritance, students often assume that there are only 2 possible alleles for a trait. This is true in some cases, but in many cases, there are more alleles for a trait. In cat-coat-color genetics, 3 different alleles of 1 gene determine the position of pigmentation on the body.
- Your genes determine all of your characteristics, and cloned organisms are exact copies of the original. While genes play a huge role in how an organism develops, environmental factors also play a role. Epigenetics is the study of heritable changes that occur without changes in the genome. The gene expression in identical mice has shown changes from factors such as diet and exposure to toxins. Further studies with identical twins have suggested that these changes can accumulate over the life of the organism. The cloning of Rainbow, a domestic cat, demonstrated 1 striking example of epigenetics. Rainbow’s coat showed calico coloration, while the coat of the clone, named Copycat, is a tabby pattern. Because Copycat and Rainbow had identical genomes, the differences must be due to epigenetic factors.
- All mutations are harmful. A mutation is a change in the genetic code of an organism. Many mutations are harmful and cause the organism not to develop properly. However, many mutations are silent and some prove beneficial. In the case of a silent mutation, the change in the genome does not change the production of the amino acid sequence and subsequent protein (remember that multiple codons may code for the same amino acid, so a change in 1 nucleotide does not necessarily change the gene product). If an organism does live with a mutation, then often the environment will determine whether the mutation is beneficial or harmful. Production of 1 protein vs. another may confer a characteristic such as a difference in coloration or in the ability to digest a resource (e.g., the ability to digest lactose or maltose instead of sucrose). The phenotypic outcome may be selected, for or against, depending on environmental factors.
- A dominant trait is the most likely to be found in the population. The term “dominant allele” sometimes conveys to students the impression that the allele is the one that exists in the greatest proportion in a population; however, “dominant” refers only to the allele’s expression over another allele. Human genetics includes examples of dominant traits that do not affect the majority of the population. In fact, achondroplasia, a type of dwarfism caused by the presence of a dominant allele, is found in fewer than 1 in 10,000 live births. Huntington’s disease, a degenerative disease caused by the presence of a dominant allele, occurs at a rate of about 3 to 7 cases per 100,000 people of European descent.
- Genetics terms are often confused. Many students understand the basic ideas of genetics but need more familiarity with the terms. For example, students often struggle with the difference between a chromosome, a gene, and an allele. Chromosomes are organized structures containing proteins and a single coiled strand of DNA; chromosomes are visible with a microscope only during parts of the cell cycle. Genes are units of heredity—specific sequences of DNA or RNA that create proteins with particular functions in an organism. Alleles are variants of a gene. Making sure that students have a strong foundation in the terminology can greatly improve their understanding of genetics and prevent misconceptions.
Dispelling these 5 misconceptions will help students better understand genetics information and activities that you plan for both the classroom and the lab. They will also realize there are many influences on the way living things develop genetically over time.